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Thus, the eye, the ear Post Date: Tue, 19 Aug 2008 4:16:34 +0000 ) Thus, light acts as a stimulus only on the eye, sound waves only affect the ear, odoriferous substances the nose, flavours the tongue, warmth and pressure the skin. From these organs the stimuli are conveyed by nerves to the brain, where they are manifested in our consciousness. Thus, the eye, the ear, the tongue, and the skin are the instruments of the senses (of sight, hearing, etc. Autor of the post: Undefined 5 Respiration Post Date: Tue, 19 Aug 2008 4:06:16 +0000 ), or, in other words, implements or organs of sense. In the vertebrata these organs exhibit a highly-perfected development. 5 Respiration. Autor of the post: Undefined If we examine the air Post Date: Tue, 19 Aug 2008 3:48:27 +0000 No animal can continue to exist if its respiration is for any length of time interrupted ie, if it is prevented from taking in fresh air. Nor is the kind of air so taken up to be left out of considera- tion, for any animal will speedily die of suffocation in an air not con- taining oxygen gas. If we examine the air which has been breathed out (eg, by man), we shall find that a portion of the oxygen has disappeared (prove this by showing that a flame burning in a vessel containing such air is extinguished much sooner than when burning in a vessel filled with atmospheric air), and this portion of oxygen has been replaced by carbonic acid gas. Autor of the post: Undefined The exchange of the two Post Date: Tue, 19 Aug 2008 3:32:49 +0000 (Prove by breathing into lime-water.) Hence, respiration consists in the consumption of oxygen and in the production of carbonic acid gas, and is accordingly a kind of combustion. The exchange of the two gases is effected either by means of lungs or gills, according as the animal derives its oxygen direct from the atmo- sphere or from atmospheric air dissolved in water. Autor of the post: Undefined (The upper part Post Date: Tue, 19 Aug 2008 3:21:59 +0000 Let us explain this process of respiration more fully in a mammalian animal breathing by means of lungs. The air is taken in through the nose (or mouth), and conducted through the trachea to the huiys. (The upper part of the trachea is formed by the larynx, which is protected against the entrance of food by means of a lid or cover. Autor of the post: Undefined ) The tracheal tubes ramify Post Date: Tue, 19 Aug 2008 3:04:37 +0000 Two bands vocal chords are stretched across the larynx, and are made to vibrate by means of currents of air of sufficient strength issuing from the lungs. Tones are thus produced in exactly the same manner as in a reed-pipe.) The tracheal tubes ramify in the lungs like the branches of a tree, and terminate in a very large number of very minute air-cells, the pulmonary vesicles. Autor of the post: Undefined Now, from an easily performed Post Date: Tue, 19 Aug 2008 2:52:53 +0000 Each of these extremely fine air-cells is surrounded by a net- work of bloodvessels of hair-like fineness (capillaries}, which contain blood very rich in carbonic acid. Two kinds of air, oxygen and carbonic acid gas, are thus separated by the fine wall of these air-bladders. Now, from an easily performed experiment, we learn that if two chambers which are separated by an animal membrane are filled with different gases, an exchange of the latter takes place through the membrane until the two gases become completely mingled. Autor of the post: Undefined 6 Circulation of the Blood Post Date: Tue, 19 Aug 2008 2:35:00 +0000 In the same manner a mutual exchange between the oxygen and carbonic acid gas takes place in the pulmonary air-cells. The carbonic acid gas is expired, while the oxygen is taken up and carried off by the blood. 6 Circulation of the Blood . Autor of the post: Undefined )' The heart is a hollow Post Date: Tue, 19 Aug 2008 2:15:50 +0000 The capillaries of the lungs unite into larger and larger tubes, the pulmonary veins, which convey the bright-red blood to the heart. (In the fishes only, which see, is the blood conducted directly from the gills to the body.)' The heart is a hollow muscle, divided into separate compartments, which drives the blood in rhythmic beats or pulsations, after the manner of a pump (the beat of the heart), through the arteries all over the body. Autor of the post: Undefined Through the walls Post Date: Tue, 19 Aug 2008 2:02:55 +0000 The rhythmic beats of the heart are rendered manifest in these vessels, as the pulse. The large arteries divide up into finer and finer branches, and finally, as in the lungs, terminate in capillaries, which penetrate and enmesh all the parts of the body. Through the walls of the capillaries an exchange of gases is effected in a reverse direction to that in the lungs. 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